Commercially available DC-to-AC (DC/AC) converters, e.g., 12 VDC to 120 VAC root-mean-square (RMS), 60 Hz converters, frequently have difficulty starting various electrical devices, e.g., televisions, motors, etc., that draw heavy currents when the devices are first powered. Further, commercially available DC/AC converters may also have difficulty supplying an appropriate link voltage to achieve a desired sinusoidal voltage during various other overload conditions. In general, when heavy start-up loads are applied, the converter DC link voltage may be loaded excessively and fall below the required value. This low link voltage, in turn, results in waveform distortion at the output of the DC/AC converter, which may result in suboptimal load starting performance.
Additionally, the link voltage of a converter may also fall below a required value during operation at times other than at start-up. In commercially available DC/AC converters, when the link voltage of the converter falls during a high load current situation, peak clipping of the output waveform of the converter usually results. Converters that are designed to maintain a RMS voltage or average voltage typically respond to the falling peak voltage by increasing a gain factor, to boost the non-peak portion of the output waveform, in an attempt to maintain the desired RMS voltage.
Unfortunately, the dV/dt (rise-time) in such a converter may become abnormally high, which can cause problems for some types of loads, such as those that rectify AC and charge a capacitor. In addition, commercially available DC/AC converters have generally not managed harmonic content of the output waveform, which can be problematic for magnetic devices, such as transformers and motors. Typical DC/AC converters have used a DC-to-DC (DC/DC) converter to boost a system voltage, e.g., 14 Volts DC, to a voltage above the peak of a desired sinusoidal output voltage, e.g., 170 Volts for a 120 Volt AC system. In such DC/AC converters, while an associated inverter is capable of delivering more power than its continuous rating for a brief period of time, subject to thermal limitations, the DC/DC converter is often not capable of sustaining the desired link voltage with the increased power delivery. Consequently, such DC/AC converters have required that the DC/DC converter be oversized in order to start or maintain operation of an associated load during an overload condition. Unfortunately, oversizing the DC/DC converter increases the overall cost of the DC/AC converter, which is undesirable.
What is needed is a technique for increasing the power capability of a DC/AC converter to better allow the converter to handle overload conditions that may occur at start-up or during operation of the converter.